In manufacturing and assembly processes, it is often desirable to measure an object surface with a high degree of accuracy. One technique for performing such measurements with high accuracy (e.g. on the order of microns) and low noise is to employ a coordinate measuring machine (CMM). The CMM applies a touch probe to the surface of the object, which can be mounted on a moving motion stage (or, alternatively, the touch probe can be moved). As motion occurs (e.g. in the physical x and physical y-coordinate directions) the probe contacts a number of locations along the object surface, thereby generating a map of the overall displacement (e.g. in the physical z-coordinate direction) with respect to various locations on the object surface. Since this approach requires contact with the surface, it tends to be time-consuming.
Alternatively, the displacement (or “profile”) of the object surface can be determined using a machine vision system (also termed herein “vision system”) in the form of a laser displacement sensor (also termed a laser beam “profiler”). A laser displacement sensor captures and determines the (three dimensional) profile of a scanned object surface using a planar curtain or “fan” of a laser beam at a particular plane transverse to the beam propagation path. In a conventional arrangement, a vision system camera assembly is oriented with an optical axis at a non-parallel angle relative to the plane of the beam. This arrangement captures the profile of the projected line (e.g. extending along the physical x-axis) on the object surface, which, due to the relative angle between the beam (fan) plane and the camera optical axis causes the imaged line to appear as varying in the image y-axis direction as a function of the physical z-axis height of the imaged point (along the image x-axis). The camera typically resides above the surface plane and the camera lens axis resides at an acute angle (i.e. off-axis) relative to the plane so as to capture the deviation of the laser line upon the surface. This deviation represents the profile of the surface. Laser displacement sensors are useful in a wide range of inspection and manufacturing operations where the user desires to measure and characterize surface details of a scanned object via triangulation. One form of laser displacement sensor uses a vision system camera having a lens assembly and image sensor (or “imager”) that can be based upon a CCD or CMOS design. The imager defines a predetermined field of grayscale or color-sensing pixels on an image plane that receives focused light from an imaged scene through a lens.
In a typical arrangement, the displacement sensor and/or object are in relative motion (usually in the physical y-coordinate direction) so that the object surface is scanned by the camera, and a sequence of images are acquired of the laser line at desired spatial intervals—typically in association with an encoder or other motion measurement device (or, alternatively, at time based intervals). Each of these single profile lines is typically derived from a single acquired image. These lines collectively describe the surface of the imaged object. While this approach is faster than a CMM, it tends to suffer from random noise due, for example, to laser speckle, which can reduce physical z-height measurement accuracy at a single point to the range of 100 microns RMS. This and other challenges (described further below) limit the accuracy and applicability of a conventional laser displacement sensor.